The invention relates to a mounting for a quartz crystal, which is configured as a disc and has metal layers on both sides as electrical contacting surfaces, and to a quartz crystal microbalance which includes such a mounting and a plug-in module adapted thereto.
The sensitive detection of alterations in mass with the aid of quartz crystal microbalances has been used successfully for several years already. For example such a measuring technique is used in the application of methods of surface coating, for example in the vapour deposition of metals or metal oxides. The method is based on the knowledge that a mass applied to a quartz crystal oscillating at its own natural frequency causes an easily measurable alteration in the oscillation frequency of the quartz with the frequency shift, alterations in mass which correspond to only a few atomic positions of the applied material can easily be detected. According to the quartz crystal used, the natural frequency of the quartz ranges from roughly 1 MHz to roughly 10 MHz. The frequency alteration which can here be identified, and which is in a large frequency range proportional to the amount of material applied, can be determined very exactly. The alteration in mass corresponding to the alteration in frequency can be calculated according to Sauerbrey""s equation:
xcex94f=xe2x88x92[2fo Q(A xcexcQxcfx81Q)xe2x88x921xcex94m]
here
fo is the measurable frequency alteration on the basis of the alteration in mass xcex94m of the quartz,
fo is the natural frequency of the quartz without additional mass,
A is the geometric electrode surface of the quartz,
xcexcois the modulus of rigidity of the quartz and
xcfx81o is the density of the quartz.
The principle of this measurement is based on the fact that the synthetic piezoelectric quartzes used when electrically excited in the megahertz range carry out a shear vibration. The vibration frequency depends on the thickness of the quartz and on the additional mass applied to the quartz, for example a vapour-deposited metal layer.
For roughly 35 years, the measurement of the alteration in mass xcex94m with the quartz crystal microbalance has been used not only with coating methods in vacuum but also in electrochemical experiments in which one side of the quartz crystal is in complete contact with a liquid xe2x80x9cThe Quartz crystal microbalance: A Novel Approach to the In-Situ Investigation of Interfacial Phenomena at the Solid/Liquid Junctionxe2x80x9d by R. Schumacher in Angew. Chemie, Int. Ed., volume 29 (1990), pages 329 to 438). In this application, up to now predominantly scientific questions have been in the forefront, which arise during the investigation of interfacial reactions in liquid media. Together with further investigation methods, for example the measurement of the differential capacitance and the charge flow through the interface, this technique makes possible greater insight into interfacial reactions. For example this technique is sensitive enough also to detect the presence of absorbates on the quartz.
The quartz crystals are manufactured as thin wafers which are excited with an electronic oscillator circuit in shear vibrations parallel to the surface of the wafers. In order to transfer the excitation vibration to the quartz, contacting electrodes are applied to the quartz surfaces. For this purpose, large-area metal layers are for example vapour-deposited or sputtered onto both surfaces of the wafer, which layers are electrically connected to the exciting circuit. The oscillation frequency of the quartz is registered by means of standard measuring methods. For example in the essay by R. Schumacher, ibid., references are made to suitable oscillator circuits.
In U.S. Pat. No. 4,561,286 is described a piezoelectric contamination detector which is used in a gaseous environment. A quartz crystal is used in the detector. The quartz crystal is contacted via contact springs which are in contact with the contacting surfaces on the quartz.
In U.S. Pat. No. 4,917,499 is also described a device for analysing contamination in a gaseous environment, in which device the quartz crystal used is contacted electrically via springs. In this publication it is mentioned that these springs are used in order to achieve even distribution of the forces acting on the quartz, and thus to avoid the formation of tensions in the quartz.
In contrast to applications in which the quartz crystals are used in a gaseous environment, specific problems arise when quartz crystals are used which are in contact with liquid: on the one hand, care must be taken that no liquid reaches the electrical leads to) the contacting electrodes on the quartz and certainly does not come into contact with the electronic exciting circuit. Simultaneously, however, it must also be guaranteed that the excitation of the quartz vibrations is not hindered by the mounting of the quartz (R. Schumacher, ibid.). Simultaneously satisfying these two requirements is frequently difficult, since expediently mountings are used in which the quartz is not glued in, but in which the liquid-tightness is intended to be achieved by means of easily fitted and detachable sealing means, for example toroidal sealing rings to seal against penetrating fluid. The quartz is mechanically fixed with these sealing means so that the oscillation can be prevented.
In U.S. Pat. No. 5,201,215 is described a quartz crystal microbalance, in which the quartz crystal is in contact with a liquid and is contacted via securely connected contacts.
What has proved to be problematic is that the ability of the quartzes to start vibrating in a liquid is much lower than when used in a gaseous environment or in vacuum. In the latter case, on the other hand, the quartzes can be easily excited. If the quartzes are dipped into a liquid, the latter acts like a brake and dampens the shear vibration. The measures taken to prevent liquid from penetrating into the interior of the measurement cell in addition prevent the excitation of the quartz, such that the quartz vibration can altogether become easily unstable.
In particular in the monitoring of electroplating baths, metal s constantly deposited on the quartz crystal such that the deposited metal has to be removed intermittently again and again from the quartz. During the quick dissolution process desired for this purpose, the vibration excitation regularly breaks down, such that the vibration excitation has to be started up again once the dissolution process is completed. After several cycles of these deposition and subsequent dissolution processes, the quartzes have to be regularly exchanged in order to apply fresh metal layer electrodes to both sides of the quartz for renewed contacting. During the renewed fitting of the quartz, notice must be taken both of the low mechanical stability of the quartz wafers, the liquid-tightness of the measuring apparatus against penetrating liquid and of the reproducible vibration excitation of the quartzes. From these points of view the previously available techniques for fitting the quartz wafers do not guarantee any problem-free exchange since at least some of the above-mentioned problems almost always arise.
The problem underlying the present invention, therefore, is to avoid the disadvantages of the previously known quartz crystal microbalances and especially to find a device with which reproducible vibration excitation of the quartz can be also guaranteed under the technical conditions which prevail when it is used for monitoring electroplating baths.
Above all it must also be ensured that the bath liquid does not penetrate into the device, that the quartz crystal is easily exchangeable and that, after a sufficiently long period of time during which metal is deposited on the contacting surfaces, it can be freed of deposited metal again without any problem by means of an electrolytic method.
This problem is solved by the mounting according to the invention and according to claim 1 and the quartz crystal microbalance according to claim 11. Preferred embodiments of the invention are quoted in the subordinate claims.
The device according to the invention serves as a mounting for a quartz crystal which s configured as a disc and has metal layers on both sides as electrical contacting surfaces. Essential components of this mounting are two holding elements which are detachably connected to one another as a form-fit. Moreover, at least two contact elements, brought into electrical contact with the contacting surfaces of the quartz crystal, are provided on the holding elements. The quartz crystal is fixed with the holding elements and/or the contact elements. At east one of the contact elements is designed as a resilient body. In order to solve the above-mentioned problems, the at least one resilient contact element has an end face for electrically contacting the quartz crystal, the size of which face corresponds roughly to the size of the quartz crystal.
Electrical leads are provided to the contact elements.
The mounting is used in particular in a quartz crystal microbalance which can be used in the sense described initially for scientific investigations of electro-chemical processes on interfaces and for monitoring coating processes, for example in electroplating methods, but also in vacuum-coating methods. The quartz crystal microbalance is formed by the mounting according to the invention, including the feature that at least one of the contact elements is designed as a resilient body, and by an additional plug-in module with which the mounting can be detachably connected. The plug-in module has an electronic oscillator circuit for exciting the quartz crystal with its natural frequency. In addition, electrical leads are provided to the contact elements from the oscillator circuit via plug contacts between the mounting and the plug-in module.
With the mounting according to the invention it is possible for the first time to fit quartz crystals in a mounting so as to be easily exchangeable, with the proviso that the quartz oscillations can be generated reproducibly, even after metal deposited on the quartz crystal has been removed again quickly in an anodic dissolution process, and even after such deposition and dissolution processes have been carried out several tries.
It is assumed that the problems observed with the known devices could be traced back to insufficiently good electrical contacting of the containing surfaces on the quartz crystal during the fitting of the quartz. By now using a resilient body as the contact element, which has an end face for electrically contacting the quartz crystal, this face being of a size which corresponds roughly to the size of the quartz crystal, a large-area electrical contact between the contacting surfaces on the quartz crystal and the contact elements can be reproducibly produced. Even when minimal inaccuracies occur during the fitting of the quartz crystal which lead to tilting of the contact elements in relation to the contacting surfaces on the quartz, secure support of the contact elements on the contacting surfaces of the quartz crystal is guaranteed. Through the arrangement according to he invention, a form-fit detachable connection of the holding elements can also be selected instead of, for example, a glue connection, since obviously inaccuracies in fixing the quartz do not lead to a worsening of the electrical contacting. This is based probably on the fact that the resilient contact element can compensate for tolerances.
By a plug-in module being used for the quartz crystal microbalance, with which module the mounting can be detachably connected, what can furthermore be achieved is that the quartz crystals can be quickly exchanged in a problem-free manner, if a quartz crystal is no longer usable after repeated deposition and dissolution processes. Thus the increased assembly outlay For a quartz crystal during a process of exchanging the quartz in the quartz crystal microbalance for another does not have to be afforded. In the chosen conception, the plug contact includes the electronic circuit required for the vibration excitation of one quartz. Thus when the quartz is changed the circuit does not also have to be exchanged; therefore only one electronic circuit is required. As a result of the low spatial distance between the quartz crystal and the exciting circuit in the plug-in module, the oscillation stimulation behaviour is further improved.
The resilient body is preferably designed as a spring with end faces which are parallel to one another and serve the electrical contacting of the quartz crystal. This spring can include at least one central and two outer rings and/or discs arranged in stacks, respectively two rings and/or discs being connected to one another via at least one web, and the webs between two rings and/or discs being offset in relation to webs between adjacent rings and/or discs.
In a particularly preferred embodiment, the spring has a central and two outer rings and/or discs, arranged in stacks, respectively two rings and/or discs being connected to one another via two webs facing one another, and the two webs between the central and the one outer ring and/or the disc being arranged offset in relation to the two webs between the central and the other outer ring and/or the disc by respectively roughly 90xc2x0.
With this design of the resilient body, an extensive balancing of the tilting forces at the spring is achieved. With conventional springs, in contrast to this there is the disadvantageous tendency that tilting forces are not uniformly large in all directions. This means that secure support of the end faces of the contact elements on the contacting surfaces of the quartz crystal can no longer be easily guaranteed.
The balancing of tilting forces acting on the end faces of the spring is achieved all the more, the more webs are provided between two rings and/or discs. However, the spring constant is also greater as the number of webs increases, such that there is an optimum of two webs between two rings and/or discs.
In an alternative embodiment, the spring can include at least two rings and/or discs arranged in stacks and which are respectively connected to one another by at least two webs, all the webs between two rings and/or discs forming an equal angle of between roughly 10xc2x0 and roughly 80xc2x0 to the spring axis. By setting an angle of between roughly 10xc2x0 and roughly 80xc2x0 between the webs and the spring axis, and through the choice of the material and the material thickness of the webs, the spring constant is adjusted. With this structure, too, an extensive balancing of the spring against tilting forces is achieved.
In a further embodiment, the resilient body can also be designed as a hydraulic element with end faces which are parallel to one another and which serve the electrical contacting of the quartz crystal. For example, the element could be designed as a hollow toroidal sealing ring, which is filled with gas or liquid and which is disposed between two rings and/or discs with the end faces serving the contacting. Alternatively, the resilient contact element could also include two discs, of which one has a cavity filled with gas or liquid, in which precision bores distributed evenly over the disc surface are introduced with dies fitted therein. The second disc rests in this case against the hydraulically moveable die.
For easy and quick fixing of the quartz crystal, the mounting is so realised that the holding elements are screwed to one another or are connected to one another by a bayonet lock. To exchange the quartz, in this case the holding elements are merely screwed away from one another or the bayonet lock is released, the old quartz crystal is taken from the device, and after a new quartz has been fitted, the holding elements are screwed to one another again or the bayonet lock is closed. Furthermore, the holding elements can also be connected to one another by means of a snap closure.
To use the mounting in a quartz crystal microbalance, one side of the quartz crystal comes into contact with the fluid medium located outside the mounting, (gas or liquid). For this purpose, the one contact element is configured disc-shaped (for the rear side contacting) and the other contact element is configured annular (for the front side contacting), the contact elements being equipped respectively with metallic end faces which stand perpendicular to the axis of the screw connection or of the bayonet lock and which serve the electrical contacting of the quartz crystal. The end face of the annular contact element is naturally also annular, such that the fluid medium in the region inside the annular opening at the end face can reach the one side of the quartz crystal.
For simple and reliable contacting of the two quartz contacting surfaces, in this case both contact elements are detachably connected to one of the holding elements. So that the fluid medium can come into contact with the one side of the quartz crystal, the holding element with which the contact elements are not connected is configured annular. The annular contact element is fitted to the side of the quartz crystal facing the annular holding element, such that fluid medium located outside the mounting can come into contact with the one side of the fixed quartz crystal.